Re: hdd + ssd

Emmanuel Florac
2015-10-23T11:48:11Z

Le Thu, 22 Oct 2015 20:23:24 +0200
"krautus@kr916.org" <krautus@kr916.org> écrivait:

> Hello I'm trying to understand why and how to add one or more SSD (as
> a cache drive / keeping xfs log) to a traditional spinning xfs
> storage volume. I mean: which data will go to the ssd ? Inodes and
> dentries will go to the ssd ? Will the _read_ performance increase ?
>
> In general I'm looking to increase (cache) the reading performance of
> folders with a lot of small files (emails), for email servers.
>
> Feel free to let me rtfm :)
> I'd gladly study the documentation / articles / benchmarks but my
> google-fu isn't in best shape.

You've got several options, some integrated with the kernel: dm-cache
and bcache, some available as additional tools: flashcache and
EnhanceIO.

YMMV, but here's my take:

* flashcache being a facebook internal dev, probably is the most
largely deployed one. It's clearly production-ready.

* EnhanceIO works fine but I haven't tested it thoroughly. It adds no
signature to the drives so it can be added to existing filesystems
(flashcache and bcache need reformatting). However that means that
bad thing may happen if you're careless -- it's clearly targeted at
always-on servers.

* bcache works fine but the latest fixes haven't been backported, so
you should probably use it only with latest (4.2, 4.3) kernels. It's
not very mature yet but it's *friggin' fast*.

* dm-cache is the easiest to set-up with the lvmcache command (if your
distro is recent enough of course). Like very very easy. It's
unfortunately the slowest of the pack, apparently. Doesn't need
reformatting IF your existing FS already lives in a LV.

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